It is important to detect a fuel injection condition, such as a fuel-injection-start timing, a fuel injection quantity and the like in order to accurately control an output torque and an emission of an internal combustion engine. Conventionally, it is known that an actual fuel injection condition is detected by sensing a fuel pressure in a fuel injection system, which is varied due to a fuel injection.
For example, JP-2008-144749A (US-2008-0228374A1) describes that an actual fuel-injection-start timing is detected by detecting a timing at which the fuel pressure in the fuel injection system starts to be decreased due to a start of the fuel injection and the fuel-injection-end timing is detected by detecting a timing at which the fuel pressure increase is stopped.
A fuel pressure sensor disposed in a common rail hardly detects a variation in the fuel pressure with high accuracy because the fuel pressure variation due to the fuel injection is attenuated in the common rail. JP-2008-144749A and JP-2000-265892A describe that a fuel pressure sensor is disposed in a fuel injector to detect the variation in the fuel pressure before the variation is attenuated in the common rail.
JP-2008-144749A shows a method of computing the fuel-injection-start timing based on a pressure waveform detected by the pressure sensor disposed in a fuel injector, which method will be described hereinafter.
As shown in FIG. 13A, when a command signal for starting a fuel injection is outputted from an electronic control unit (ECU) at a fuel-injection-start command timing “Is”, a driving current supplied from an electronic driver unit (EDU) to a fuel injector starts to rise at the fuel-injection-start command timing “Is”. A detection pressure detected by the fuel pressure sensor varies as shown by a solid line “L1” in FIG. 13B.
It should be noted that the command signal for starting a fuel injection is referred to as a SFC-signal and the command signal for ending a fuel injection is referred to as an EFC-signal, hereinafter.
When the SFC-signal is outputted from the ECU at the fuel-injection-start command timing “Is” and an injection rate (injection quantity per unit time) increases, the detection pressure starts to decrease at a changing point “P3a” on the pressure waveform.
A timing at which the changing point “P3a” appears is detected and the fuel-injection-start timing is computed based on its detection timing of the changing point “P3a”. Specifically, as shown by a solid line M1 in FIG. 13C, differential values are computed with respect to every detection pressure. After the SFC-signal is outputted at the fuel-injection-start command timing “Is”, the differential value first becomes lower than a threshold TH at a timing “t1”. This timing “t1” is detected as the timing at which the changing point “P3a” appears.
It should be noted that when the fuel injection rate starts to decrease, the detection pressure starts to increase at a changing point “P7a”. When the fuel injection rate becomes zero, an increase in the detection pressure ends at a changing point “P8a”. 
The pressure waveform illustrated by the solid line L1 in FIG. 13B represents a waveform in a case that a single fuel injection is performed during one combustion cycle. In a case that a multi-stage injection is performed, the pressure waveform generated by the second or successive fuel injection is illustrated by a broken line L2. This pressure waveform illustrated by the broken line L2 is generated by overlapping an aftermath (refer to an encircled portion “A0” in FIG. 13B) of the previous waveform with the current waveform.
As a result, the differential values shift from the solid line M1 to a dashed line M2. That is, the differential value becomes lower than the threshold TH at a timing “tx”, and this timing “tx” which is earlier than the actual fuel-injection-start timing “t1” is erroneously detected as the fuel-injection-start timing.
Especially, in a case that a multi-stage injection is performed, when an interval between n-th injection and (n+1)th injection is short, the pressure waveform of n-th fuel injection encircled by the line A0 overlaps with the pressure waveform of (n+1)th fuel injection, which increase the pulsations of the pressure waveform and the deferential values. The erroneous detection frequently occurs.
Moreover, it is conceivable that noises overlapping on the pressure waveform may cause a deformation of the pressure waveform. Thus, even in a case that single-stage injection is performed or the interval between adjacent injections is long, the above mentioned erroneous detection of the actual fuel-injection-start timing may be performed.